Telephone answering units are presently available which respond to ring signals received over the telephone line to be activated so that a recorded announcement may be transmitted over the telephone line to the calling party, and so that a message from the calling party may subsequently be recorded.
However, problems have arisen in the past in making such telephone answering units unresponsive to dialing pulses from the associated telephone, so that the unit will not respond to such pulses when the subscriber is dialing his telephone. For example, often when a person is dialing his telephone, the usual prior art telephone answering unit has a tendency to respond to the dialing pulses, mistaking them for ring signals, and causing the system to come on to the telephone line and break the outgoing dialing pulses so that the outgoing call cannot be completed.
Systems have been proposed in the past which distinguish between dialing pulses and ring signals by sensing whether or not the associated telephone is in an "on-hook" or "off-hook" condition. For example, such systems are described in U.S. Pat. Nos. 3,899,644 and 4,074,081. However, such systems are not acceptable in conjunction with present-day telephones, because of the requirement by the Federal Communications Commission that telephone answering systems be direct-current isolated from the telephone line, and such prior art systems require a direct current coupling to the telephone line, with substantial loading of the line.
Also, in some telephone systems, the direct-current voltage level of the line increases when the receiver is taken off the hook, and in other telephone systems the direct-current level decreases, so that, for commercial practicability, it is essential that the system respond both to voltage increases or voltage decreases to perform its function. The system of the present invention is constructed to respond both to voltage increases or voltage decreases on the telephone line to detect whether the associated telephone is in an "on-hook" or "off-hook" condition.
The system of the present invention, accordingly, has utility in conjunction with telephone answering units. However, it will become evident as the description proceeds that the system of the invention is not limited to such a use, but finds finds utility in conjunction with a number of systems in which incoming signals are to be distinguished, and only one type of incoming signal is to be recognized and detected.
Distinguishing between ring signals and dial pulses on the telephone line has presented a problem in the prior art systems because the ringing signals and dial pulses closely resemble one another. The ringing signal is typically a high voltage wave, which may be either square or sinusoidal in shape, with an amplitude of approximately 40-100 volts RMS, or approximately 113-282 volts peak-to-peak; and of a frequency which may range from 16 Hz to 67 Hz. The dial pulses, on the other hand, typically produce a waveform which is relatively square in shape with a maximum amplitude of approximately 100 volts peak-to-peak, and a frequency which may be within the range of normal ringing frequencies.
It is therefore, virtually impossible to differentiate between the ring signals and the dialing pulses on the telephone line on the basis of frequency or wave shape, since both may be the same. It is also virtually impossible to differentiate between the ring signals and dialing pulses on the basis of voltage amplitudes since these voltages have no established relationship, and usually are of substantially the same amplitude. Also, systems which attempt to differentiate between ring signals and dialing pulses by counting the pulses or bursts are unreliable, since these may vary from one telephone system to another.
As explained above, the system of the present invention provides a positive means for differentiating between ring signals and dialing pulses, by detecting whether or not the associated telephone line is in an on-hook or off-hook condition, and by producing a detected output only when the telephone line is in the on-hook condition, indicating that the detected signal is actually a ring signal and not dialing pulses.